1. Field of the Invention
This invention relates to an improvement of the modulated output signal of a color television signal encoder. In particular, the invention is useful to phase synchronize a frequency modulated subcarrier and a modulating signal and further to inhibit modulated signal crosstalk between the simultaneously transmitted liminance and chrominance signals of a frequency modulated carrier. This improvement is particularly applicable to color television systems operative according to the SECAM standard developed in France.
In order to appreciate the problems solved by the present invention, it is helpful to understand basic principles of operation of the principal color television standards, NTSC (United States), PAL (Western Germany) which are phase quadrature am standards, and SECAM, which is a sequential fm standard. Inasmuch as the preferred application of the present invention is used in a frequency modulated carrier system, the invention is described in the context of the SECAM standard. Where the invention is applicable in the systems of other standards, relevant operational characteristics are explained.
A color television signal comprises "luminance" (brightness) and "chrominance" (hue) information which must be encoded in such a manner as to be decodable in both compatable wideband black and white and color receivers. SECAM, an acronym for "sequential color and memory", is a color television encoding standard wherein a composite of chrominance information and luminance information is provided in frequency modulated form. The composite signal comprises: (1), in one frequency band, a frequency moudlated luminance signal equal to the sum of the weighted intensity values of the three primary hues, red, green and blue, and (2), in an adjacent, partially overlapping frequency band, sequentially alternating frequency modulated chrominance signals containing information consisting of the differences between the intensities of two primary hue values, red and blue, and the corresponding luminance signal. As sequential chrominance transmission is utilized, as opposed to simultaneous chrominance transmission, two lines are generally required to provide the signal information of a complete scan line image. As a result, a color memory device is necessary in the decoder (at the receiving end). The color memory device typically comprises a delay line.
2. Description of the Prior Art
In the past, certain parameter restrictions imposed by the SECAM standard have been extremely difficult to satisfy. Prior art signal encoders (at the transmitting end) are not always able to provide a sufficiently unambiguous signal to the receiver and signal decoders for reproducing reliably accurate television images.
For example, the SECAM standard dictates that the center frequency of the luminance signal and the center frequency of the sequential chrominance signals be exact harmonics of the horizontal line scan frequency. Furthermore, the standard requires absolute phase coincidence between the line scan frequency and the modulated chrominance signals at the beginning of each alternating line scan.
Prior art SECAM encoders, particularly encoders sold by MATRA of France (Model M380R) and by the French consortium, Thompson-CSF (Model TTV4630), have addressed the problem of satisfying the SECAM standard with marginal success. These encoders have adopted the use of a single sub-carrier modulator in conjunction with a master switching device. According to the prior art technique, the modulated chrominance signals are alternately switched and locked to the master oscillator via the master switching device at the end of each scanned line (e.g., during the blanking period).
A major problem exists with this technique in that it is extremely difficult to achieve absolute phase coherence between the master oscillator and the switched modulated chrominance signals, since so little time is provided for phase locking. In particular, the prior art single modulator/oscillator system permits phase locking only during the line blanking periods, which may be as short as 10 or 12 cycles at the sub-carrier frequency, which is typically about 4 MHz. Thus, only about 2.5 microseconds is provided for phase locking. Prior art encoders operative according to this single-oscillator/modulator technique typically require additional circuitry or special components to assure proper operation. As one example, temperature compensated high precision components may generally be required.
Another problem associated with the SECAM standard is the interference between luminance and chrominance information under certain signal conditions, which may cause receiver error in the receiver decoding circuitry. This phenomenon is generally known as crosstalk, and it is a particular characteristic of frequency modulated signals transmitted in closely adjacent or overlapping frequency bands. The phenomenon of crosstalk occurs when the transmitted video signal comprises a closely spaced repetitive pattern of large signal variations, such as a herringbone or picket fence. (Hence, the crosstalk phenomenon is often called the "picket fence" effect). Crosstalk is evidenced in the receiver when repetitive bursts of luminance information signals "capture" the chrominance decoding circuitry of the television receiver. This results in the acceptance by the receiver of luminance information as chrominance information, scrambling the received image.
In order to prevent frequency-modulated luminance signal interference with the chrominance information of the transmitted signal, an anti-crosstalk trap is included in the luminance channel. The anti-crosstalk traps used in practice in prior art encoders comprise a band reject or notch filter centered at the frequency of potential interference with chrominance information. The notch filter is coupled in parallel with an exactly complementary bandpass filter. In addition, a "clipping" or amplitude limiting circuit is coupled in series with the bandpass filter. The parallel outputs of the complementary filters in the luminance channel are typically recombined in an appropriate summing device. At low signal levels the complementary filters supposedly provide complementary signal cancellation resulting in an undistorted output signal. The trap is supposedly only activated and effective in modifying the luminance signal when the potentially interfering luminance information is of sufficiently high amplitude and duration to operate outside of a preselected clipping range, thereby warranting suppression. However ideal the theory of operation, in practice it is extremely difficult to attain truly complementary bandpass and band reject filters. As a result, undesired distortion may disturb the luminance signal channel over the entire spectrum of modulation.
One proposed improvement in the typical prior art anti-crosstalk trap, reported by R. Fessard of the Television Company of France, provides a band reject circuit which is activated by a voltage controlled resistor (FET) only if the luminance input signal level exceeds a preselected threshold. Such a circuit apparently does not require exact matching between the receiver bandpass and the encoder band reject network. In practice, such a circuit is difficult to implement, since for example practical difficulties are encountered in providing a uniform, reproducible threshold as a function of the luminance signal level.